1. Field of the Invention
The present invention relates to variable magnification optical systems having an image stabilizing function and, more particularly, to variable magnification optical systems having an image stabilizing function suited to photographic cameras or video cameras, in which a lens unit constituting part of the variable magnification optical system is moved in a direction perpendicular to an optical axis so as to optically compensate for the shaking of a picked-up image obtained when the variable magnification optical system vibrates (tilts), so that the picked-up image is maintained stable.
2. Description of Related Art
When shooting is performed with a photographing system on a running car, a flying air plane or the like moving vehicle, vibrations propagate to the photographing system, so that shaking would be caused in the picked-up image.
To prevent the occurrence of such image shaking, there have been many previous proposals for stabilizing the image formed in the optical system.
For example, in Japanese Patent Publication No. Sho 56-21133, in accordance with the output signal of a detecting means for detecting a vibration state of an optical apparatus, an optical member constituting part of the optical apparatus is moved in such a direction as to cancel the vibrating displacement of an image caused by the vibration of the optical apparatus, so that the image is maintained stable.
In Japanese Laid-Open Patent Application No. Sho 61-223819, in a photographing system provided with a variable angle prism of the refracting type arranged at the frontmost position thereof, an apex angle of the variable angle prism is varied in correspondence to the vibration of the photographing system so as to deflect an image, thereby stabilizing the image.
In Japanese Patent Publications No. Sho 56-34847 and No. Sho 57-7414, etc., an optical member, which is spatially fixed against the vibration of a photographing system, is disposed in part of the photographing system, and a prism effect produced by the optical member against the vibration is utilized to deflect a picked-up image, thereby stabilizing the image on a focal plane.
In Japanese Laid-Open Patent Applications No. Hei 1-116619 and No. Hei 2-124521, an acceleration sensor or the like is utilized to detect the vibration of a photographing system, and, in response to the detection signal obtained therefrom, a lens unit constituting part of the photographing system is vibrated in a direction perpendicular to an optical axis so as to stabilize a picked-up image.
Also, Japanese Laid-Open Patent Application No. Hei 7-128619 discloses a variable magnification optical system comprising, in order from the object side, a first lens unit of positive refractive power which is stationary during focusing and during zooming, a second lens unit of negative refractive power which has a magnification varying function, an aperture stop, a third lens unit of positive refractive power, and a fourth lens unit of positive refractive power, which has both the function of compensating for the shifting of the image with the variation of the magnification and the focusing function, wherein the third lens unit is composed of two lens sub-units, i.e., a first lens sub-unit of negative refractive power and a second lens sub-unit of positive refractive power, and the second lens sub-unit is moved in a direction perpendicular to an optical axis so as to compensate for the shaking of a picked-up image when the variable magnification optical system vibrates.
In Japanese Laid-Open Patent Application No. Hei 7-199124, in a 4-unit-type variable magnification optical system of a plus-minus-plus-plus refractive power arrangement, the entirety of the third lens unit is vibrated in a direction perpendicular to an optical axis so as to stabilize a picked-up image.
Meanwhile, Japanese Laid-Open Patent Application No. Hei 5-60974 discloses another 4-unit-type variable magnification optical system of a plus-minus-plus-plus refractive power arrangement, wherein the third lens unit is composed of a positive lens and a negative lens of meniscus form in the form of the telephoto type, thereby producing the advantage of reducing the total length of the entire system.
In general, the use of the method of stabilizing a picked-up image by disposing the image stabilizing optical system in front of the photographing system and vibrating a movable lens unit constituting part of the image stabilizing optical system so as to compensate for the shaking of the picked-up image causes a problem to arise in that the entire apparatus becomes larger in size and that an operating mechanism for moving the movable lens unit becomes complicated in structure.
Further, there is even a more serious problem, too, that when the movable lens unit is vibrated, a great amount of decentering aberrations would be produced with the result of a large deterioration of the optical performance.
In an optical system using the variable angle prism in stabilizing a picked-up image, there is a problem that the amount of decentering lateral chromatic aberrations produced during the image stabilization would increase, particularly, on the side of long focal lengths (telephoto side).
On the other hand, in an optical system in which a lens unit constituting part of the photographing system is vibrated in a direction perpendicular to the optical axis so as to stabilize a picked-up image, there is, an advantage that any additional optical unit dedicated to the image stabilization is unnecessary. However, there are problems that a surplus space has to be provided in the optical system so as to move the vibrating lens unit and that the amount of decentering aberrations produced during the image stabilization would increase greatly.
Further, in the 4-unit type variable magnification optical system described above which comprises positive, negative, positive and positive lens units, if the third lens unit is composed of a positive lens and a negative meniscus lens in the form of telephoto type for the purpose of shortening of the total length of the entire system, large decentering aberrations, particularly, distortional aberrations, are produced when the whole third lens unit is moved in a direction perpendicular to the optical axis to stabilize a picked-up image. In the case of applying such a variable magnification optical system to the apparatus for taking motion pictures, such as video cameras, there is a problem that the deformation of a picked-up image during the image stabilization becomes conspicuous.
In accordance with the present invention, a relatively small-sized, light-weight lens unit constituting part of a variable magnification optical system is moved in a direction perpendicular to an optical axis so as to compensate for the shaking of an image when the variable magnification optical system vibrates (tilts). By setting forth proper rules of design for the construction and arrangement of constituent lenses of the lens unit, the size of the entire system is minimized, the structure of an operating mechanism is simplified, and the load on a driving means is reduced, while still permitting the amount of decentering aberrations produced during the movement of the lens unit to be suppressed to a minimum. It is, therefore, an object of the invention to provide a variable magnification optical system having an image stabilizing function that is corrected well for decentering aberrations.
To attain the above object, in accordance with an aspect to the invention, there is provided a variable magnification optical system having an image stabilizing function, which comprises, in order from an object side to an image side, a fixed first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, the second lens unit and the fourth lens unit being moved to effect a variation of magnification, wherein the third lens unit has a negative lens of meniscus form concave toward the image side and an aspheric surface, and the third lens unit is movable in a direction perpendicular to an optical axis to stabilize an image.
In accordance with another aspect of the invention, there is provided a variable magnification optical system having an image stabilizing function, which comprises, in order from an object side to an image side, a fixed first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, the second lens unit and the fourth lens unit being moved to effect a variation of magnification, wherein the third lens unit has two lens sub-units of positive refractive power, one of which is fixed, and the other of which is movable in a direction perpendicular to an optical axis to stabilize an image.